The invention relates to optical communications systems, and more particularly to measuring optical characteristics of multiport optical communications components.
Optical components with multiple input and/or output ports are being used to support optical communications systems. Multiport optical components are especially important to next generation optical communications systems that use wavelength division multiplexing (WDM) to simultaneously transmit multiple carrier wavelengths over the same optical fiber. As optical component technologies have advanced, the number of multiplexed wavelengths and the number of ports per optical component have increased. For example, current optical communications systems multiplex from eight to thirty-two carrier wavelengths over the same optical fiber and future systems are expected to multiplex hundreds to thousands of carrier wavelengths over the same optical fiber.
During the development and testing of multiport optical components, it is necessary to measure the optical characteristics of each port of an optical component. Known techniques for measuring the optical characteristics of each port of a multiport optical component typically involve utilizing port-specific power meters to simultaneously measure responses to known input signals. For example, referring to FIG. 1, a prior art testing system involves injecting a known optical signal from an optical signal source 102 into a multiport component 110 and then measuring the optical characteristics of the optical signals that are output from multiple output ports of the multiport component. The optical characteristics of port-specific optical signals are measured with multiple port-specific power meters 114. Electrical signals generated by the port-specific power meters are then fed into a processor 116 and used to characterize the multiport component. Although utilizing multiple port-specific power meters works well to characterize a multiport component, each test system requires a dedicated power meter for each port that is tested. Power meters are expensive components of test systems and as the number of ports required for WDM communications systems increases, the number of power meters needed per testing system increases.
Another technique for measuring optical characteristics of a multiport optical component is disclosed in U.S. Pat. No. 6,023,358 issued to Baney and assigned to the assignee of this disclosure. The technique involves an optical interface device that utilizes the combination of a stimulus generator, multiplexers, couplers, and a signal analyzer to apply a light signal, one port at a time, to each port of the multiport optical component and to measure the response signal, one port at a time, that is output from each port of the multiport component. While the technique uses only one signal analyzer to measure the output response from each port of the multiport component, testing the ports is a serial operation that must be performed one port at a time. That is, the response from only one port can be measured without reconfiguring optical paths established by the multiplexers and couplers. As the number of ports in multiport components increases, reconfiguring optical paths becomes less desirable.
In view of the increasing port density of optical components and the disadvantages of prior art testing systems, what is needed is a highly scaleable multiport testing system that allows efficient testing of multiport optical components.
A system for measuring optical characteristics of a multiport optical device uses optical heterodyne detection and known port-specific transmission delays to simultaneously monitor multiple ports of the multiport optical device with a single receiver.
An embodiment of a system for measuring optical characteristics of a multiport optical device includes a splitter configured to split a swept optical signal into a reference signal and a test signal and a test system input, connectable to the multiport optical device, for transmitting the test signal to the multiport optical device. The test system also includes an optical combiner and a receiver. The optical combiner is connectable to the multiport optical device to receive a first portion of the test signal having a first port-specific transmission delay and to receive a second portion of the test signal having a second port-specific transmission delay. The optical combiner combines the first portion of the test signal having the first transmission delay and the second portion of the test signal having the second transmission delay with the reference signal. The receiver is connected to the optical combiner to detect a first optical heterodyne signal that is generated from the combined first portion of the test signal and the reference signal and to detect a second optical heterodyne signal that is generated from the combined second portion of the test signal and the reference signal.
An embodiment of the test system includes port-specific transmission delay units connected to ports of the multiport device. In an embodiment, each of the port-specific transmission delay units imparts a different transmission delay on an optical signal. In an embodiment, each of the port-specific transmission delay units is optically connected to the test system input.
An embodiment of the test system includes a processor connected to the receiver for selectively isolating one of the first and second optical heterodyne signals. In an embodiment, a frequency counter is optically connected to receive a portion of the swept optical signal and the frequency counter generates frequency information related to the swept optical signal. In an embodiment there is an electrical connection between the frequency counter and the heterodyne receiver for distributing the frequency information to the heterodyne receiver. In an embodiment there is an electrical connection between the frequency counter and the processor for distributing the frequency information to the processor.
Another embodiment of an optical system for testing a multiport optical device having a first port and a second port includes a tunable optical signal source or generating a swept optical signal and a splitter configured to split the swept optical signal into a reference signal and a test signal. The system includes a first port-specific transmission delay unit, optically connectable to the multiport optical device, for imparting a first port-specific transmission delay onto a first portion of the test signal that is specific to said first port and a second port-specific transmission delay unit, optically connectable to said multiport optical device, for imparting a second port-specific transmission delay onto a second portion of said test signal that is specific to the second port. The system includes an optical combiner and a heterodyne receiver. The optical combiner is optically connectable to the multiport device under test to combine the delayed first portion of the test signal and the delayed second portion of the test signal with the reference signal. The heterodyne receiver is connected to the optical combiner to detect a first optical heterodyne signal that is generated from the combined delayed first portion of the test signal and the reference signal and to detect a second optical heterodyne signal that is generated from the combined delayed second portion of the test signal and the reference signal.
A method for measuring optical characteristics of a multiport optical device includes steps of splitting a swept optical signal into a reference signal and a test signal and then splitting the test signal into a first port-specific portion that is specific to a first port of the multiport optical device and into a second port-specific portion that is specific to a second port of the multiport optical device. A first port-specific transmission delay is imparted onto the first port-specific portion of the test signal and a second port-specific transmission delay is imparted onto the second port-specific portion of the test signal. The delayed first port-specific portion of the test signal is combined with the reference signal to generate a first optical heterodyne signal and the first optical heterodyne signal is detected. The delayed second port-specific portion of the test signal is combined with the reference signal to generate a second optical heterodyne signal and the second optical heterodyne signal is detected. A port-specific optical characteristic of the multiport optical device is determined from one of the first and second detected optical heterodyne signals. In an embodiment, one of the first and second detected optical heterodyne signals is electronically isolated from the other signal.
An advantage of the test system is that the port-specific signals are differentiated by their transmission delay and therefore a device under test with multiple outputs can be characterized utilizing a single heterodyne receiver. Because the number of receivers does not change as the number of outputs increases, the test system is highly scaleable.